USGS Coastal and Marine Geology Program > Pacific Coastal & Marine Science Center > Research Projects > Coastal Aquifer Project: Submarine Groundwater Discharge > Methods & Tools > Measuring Resisitivity to Gauge Submarine Groundwater Discharge
Stationary, time-series, multi-electrode resistivity profiles across a beach face as a function of water level. Inversion parameters were held constant for each time step. To highlight subsurface resistivity change at the land / sea interface, images reflect only the first 40 meters of data. Inset graph of water level data collected from a Solinist Diver midway down cable, at electrode 28.
Electrical resistivity measurements can detect variations in the salinity (conductivity = 1/resistivity) regime of subsurface pore water and porous sediments. Resistivity data can be collected under two scenarios, 1) time series, and 2) streaming survey modes. In streaming survey mode, the 120-meter, 8-channel streamer system collects instantaneous dipole-dipole measurements at approximately 3-second intervals. The 120 m electrode cable, which is towed along the water's surface at a speed of about 3-4 knots, consists of two current-producing graphite electrodes and nine stainless steel electrodes spaced 10 meters apart. The depth of penetration for such a system is roughly one-third of the total streamer cable length. By merging the ship's navigation (latitude, longitude and depth) and an in-situ hydrographic data stream (salinity, pH, conductivity, temperature) with the resistivity data, inversion models can be run to process resistivity cross-sections.
Time-series resistivity for our projects is collected using an advanced Geosciences SuperSting R8 resistivity receiver and a custom 112-meter-long, high-resolution underwater cable that consists of 56 graphite electrodes spaced two meters apart in a dipole-dipole array. This cable is deployed along the shore face and seabed using a combination of electrode stakes and sandbags. Resolution of sharp salinity boundaries can be increased by using a starting model with the apparent resistivity pseudo-section overlain by water column data based on bathymetry and two conductivity, temperature, and pressure sensors. The best fitting layered model is then found by using an iterative least squares smooth model inversion method. Results from such time series resistivity measurements are compared relative to water levels.
Multi-channel resistivity investigations of the fresh water / saltwater interface: A new tool to study an old problem
Abstract in "A New Focus on Groundwater–Seawater Interactions," IAHS Publ. 312, 2007
This is a four-page full-color discussion of methods and tools in use in the study of submarine groundwater discharge in Biscayne Bay, Florida. Among the tools discussed are streaming resistivity profiling, electromagnetic seepage meters, and near-continous 222Rn surveys.